Interactions between interfaces dictate stimuli-responsive emulsion behaviour

Rey, Marcel; Kolker, Jannis; Richards, A. James; Malhotra, Isha; Glen, S. Thomas; Li, Denise Ying Ngai; Laidlaw, Fraser; Renggli, Damian; Vermant, Jan; Schofield, B. Andrew; Fujii, Syuji; Löwen, Hartmut; Clegg, Paul;

Abstract

Stimuli-responsive emulsions offer a dual advantage, combining long-term storage with controlled release triggered by external cues such as pH or temperature changes. This study establishes that thermo-responsive emulsion behaviour is primarily determined by interactions between, rather than within, interfaces. Consequently, the stability of these emulsions is intricately tied to the nature of the stabilizing microgel particles - whether they are more polymeric or colloidal, and the morphology they assume at the liquid interface. The colloidal properties of the microgels provide the foundation for the long-term stability of Pickering emulsions. However, limited deformability can lead to non-responsive emulsions. Conversely, the polymeric properties of the microgels enable them to spread and flatten at the liquid interface, enabling stimuli-responsive behaviour. Furthermore, microgels shared between two emulsion droplets in flocculated emulsions facilitate stimuli-responsiveness, regardless of their internal architecture. This underscores the pivotal role of microgel morphology and the forces they exert on liquid interfaces in the control and design of stimuli-responsive emulsions and interfaces.